Change Startup File to Default
Blender stores its configuration and startup files in a directory specific to the user. This differs depending on the operating system.

For example, on Linux this would be something along the lines of :

/home/rich/.config/blender/2.76/config


On Windows it would be something similar to :

c:\Users\rich\AppData\Roaming\Blender Foundation\Blender\2.76\config


Obviously the user ID ('rich') and the Blender version (2.76) would be different depending on your setup.

When you save the startup file, Blender saves the file in the configuration directory mentioned above. If there is no startup file when Blender starts then you'll get the 'factory default' startup file (ie, the default cube). If, however, blender finds a file named 'startup.blend' in the config directory then that will be loaded as the default state.

Therefore, to reset back to the 'factory default' startup file you simply need to locate the config directory and remove the 'startup.blend' file.

For example, on my (Linux) environment I have the following files in the 'config' directory :

(/home/rich/.config/blender/2.76/config)
-rw-rw-r-- 1 rich rich    308 Dec 14 12:45 bookmarks.txt
-rw-rw-r-- 1 rich rich    441 Jan 18 10:41 recent-files.txt
-rw-rw-r-- 1 rich rich 371104 Jan 25 11:39 startup.blend
-rw-rw-r-- 1 rich rich 116252 Sep  8  2016 userpref.blend


Removing the 'startup.blend' will reset blender back to the default startup file.